1. Field of the Invention
The present invention relates to an ultrasonic receiving apparatus and an ultrasonic receiving method to be used for receiving ultrasonic waves to obtain ultrasonic images.
2. Description of a Related Art
Conventionally, in an ultrasonic imaging apparatus, a one-dimensional sensor array using a piezoelectric element including a piezoelectric ceramic represented by PZT (Pb(lead) zirconate titanate) or a macromolecule piezoelectric element such as PVDF (polyvinyl difluoride) has been generally used as an element (vibrator) for transmitting and receiving ultrasonic waves. Two-dimensional images in plural cross-sections of an object to be inspected are obtained while mechanically shifting a one-dimensional sensor array as described above, and further, by synthesizing these two-dimensional images, a three-dimensional image is obtained.
However, according to this technique, since a time lag is generated in the shifting direction of the one-dimensional sensor array, cross-sectional images at different time points are synthesized resulting in blurred synthesized image. Accordingly, the technique is not suitable to such a case where images of a living organism as an object are taken in ultrasonic echo observation or the like.
In order to obtain high quality three-dimensional images using ultrasonic waves, a two-dimensional sensor capable of obtaining two-dimensional images without shifting the sensor array is required.
However, although minute processing on elements and wiring to a number of minute elements are required in the case where the two-dimensional sensor array is manufactured using the above-described PZT or PVDF, further miniaturization and integration of elements exceeding the state of the art are difficult. Also, even when the above-described problems are solved, such problems still remain that the cross talk between elements is increased, the S/N ratio is lowered due to increase of electrical impedance caused from minute wirings, electrodes of minute elements get damaged easily, and so on. Therefore, it is difficult to achieve two-dimensional sensor array using the PZT or the PVDF.
On the other hand, another type of sensor is also known, in which received ultrasonic wave signal is converted into an optical signal and then detected. As for a photo-detection type ultrasonic sensor, a sensor in which a fiber Bragg grating (abbreviated as FBG) is used (see TAKAHASHI et al., National Defense Academy xe2x80x9cUnderwater Acoustic Sensor with Fiber Bragg Gratingxe2x80x9d, OPTICAL REVIEW Vol.4, No.6 (1997) p.691-694), and a sensor in which a Fabry-Perot resonator (abbreviated as FPR) structure is used (see UNO et al., Tokyo Institute of Technology xe2x80x9cFabrication and Performance of a Fiber Optic Micro-Probe for Megahertz Ultrasonic Field Measurementxe2x80x9d, T.IEE Japan, Vol. 118-E, No.11, (1998) p.487-492) are reported. When a two-dimensional sensor array is manufactured by using such an ultrasonic sensor as described above, the following advantages can be obtained, that is, electrical wiring to a number of minute elements is not required and satisfactory sensitivity is obtained.
Further, a photo-detection type ultrasonic sensor having a two-dimensional detection surface is also known. For example, Beard et al., University College London xe2x80x9cTransduction Mechanisms of the Fabry-Perot Polymer Film Sensing Concept for Wideband Ultrasound Detectionxe2x80x9d, IEEE TRANSACTIONS ON ULTRSONICS, FREROELECTRICS, AND FREQUENCY CONTROL, Vol.46, No.6, November1999, p.1575-1582 discloses that a polymer film having a Fabry-Perot structure is used for detecting ultrasonic waves. In a film-like ultrasonic sensor as described above, since processing on a number of minute elements is not required, the cost can be reduced. The photo-detection type ultrasonic sensor utilizes an ultrasonic detecting element having light reflection characteristics which are changed by receiving ultrasonic waves. Herein, the term xe2x80x9creflection characteristicsxe2x80x9d means the relationship of the reflection intensity of the light with respect to the wavelengths of the light.
However, in the ultrasonic detecting element as described-above, since the reflection characteristics of the light changes due to temperature or humidity changes, fluctuation of detection sensitivity is large. Also, in the ultrasonic detecting element having a two dimensional detection surface, the light reflection characteristics differ depending on the position of the detection surface resulting in fluctuation of the detection sensitivity. Thus, in an ultrasonic receiving apparatus to which the optical detection method is applied, it is a critical problem in practical use to control the changes or fluctuation of the detection sensitivity due to ambient factor such as temperature or structural factor. In order to solve this problem, for example, it is conceivable to adjust the wavelength of light output from a light source to a point where the sensitivity of the ultrasonic detecting element is high. However, it is difficult to adjust the wavelength of the light output from the light source with respect to extremely steep reflection characteristics. On the other hand, such method is also conceivable that a broadband light is allowed to enter ultrasonic detecting elements having different reflection characteristics depending on the position and the reflected light is separated by a filter. However, in this case, such problem remains that the structure of the ultrasonic detecting elements becomes complicated resulting in a higher cost. Furthermore, although such method is also conceivable that the reflection characteristics are made to be different from each other depending on each detection area of the ultrasonic detecting element, in this case also, the structure of the ultrasonic detecting element becomes complicated resulting in a higher cost.
In order to increase the detection sensitivity in the ultrasonic receiving apparatus as described above, for example, it is conceivable to increase the inclination of the optical reflectance in the light reflection characteristics of the ultrasonic detecting elements, or to increase the compliance of an ultrasonic sensor upon receiving the ultrasonic wave. However, when the inclination of the optical reflectance in the reflection characteristics is increased, a strict adjustment accuracy of the wavelength of the light used for detection is also required. On the other hand, in order to increase the compliance of the ultrasonic sensor, a softer member may be used for the ultrasonic receiving surface of the ultrasonic sensor. However, owing to this, temperature-dependency of the reflection characteristics in the ultrasonic sensor is also increased resulting in an unstable detection operation.
The present invention has been achieved in view of the above-described problems. An object of the present invention is, in an optical detection type ultrasonic receiving apparatus and an ultrasonic receiving method, to increase the S/N ratio and the like of the detection signal while preventing the apparatus from getting larger in size and the cost thereof from increasing.
In order to solve the above-described problems, an ultrasonic receiving apparatus according to the present invention comprises a light source for generating broadband light, an ultrasonic detecting element including an ultrasonic sensing portion which is expanded and contracted by a received ultrasonic wave to change an optical reflectance thereof in accordance with expansion and contraction thereby performing intensity modulation of the light generated by the light source, spectrum separating means for spectrum-separating the light intensity-modulated by the ultrasonic detecting element, photo detecting means having a plurality of photoelectric converting elements for detecting the light spectrum-separated by the spectrum separating means for each of the plural wavelength components to generate at least a first detection signal obtained by detecting a first wavelength component and a second detection signal obtained by detecting a second wavelength component, and processing means for carrying out processing operation using the first detection signal and the second detection signal so as to obtain information about the ultrasonic wave received by the ultrasonic detecting element.
Herein, the above-described spectrum separating means may spectrum-separate the plural light beams guided from the plural detection areas of the ultrasonic detecting element in bulk. Also, the above-described photo detecting means may detect the plural light beams which are spectrum-separated by the spectrum separating means with respect to the respective wavelengths.
An ultrasonic receiving method according to the present invention comprises steps of (a) allowing light to enter an ultrasonic detecting element including an ultrasonic sensing portion which is expanded and contracted by a received ultrasonic wave to change an optical reflectance thereof in accordance with expansion and contraction thereby performing intensity modulation of the incident light, spectrum-separating the light intensity-modulated by the ultrasonic detecting element and detecting the spectrum-separated light for each of plural wavelength components by using photo detecting means having a plurality of photoelectric converting elements to generate at least a first detection signal obtained by detecting a first wavelength component and a second detection signal obtained by detecting a second wavelength component with respect to each of plural detection areas of the ultrasonic detecting element, and (b) carrying out processing operation using the first detection signal and the second detection signal so as to obtain information about the ultrasonic wave received at each of the plural detection areas of the ultrasonic detecting element.
Herein, step (b) may include adding the first detection signal and the second detection signal to each other after inverting the phase of either one thereof.
According to the present invention, since the processing is carried out on the basis of the plural detection signals obtained by detecting the plural wavelength components of the modulated light by using the ultrasonic detecting element, the S/N ratio or the like of the detection signals can be increased.